Conventional automotive vehicles, such as automobiles, trucks and buses, typically include a heating system for supplying warm air to a passenger compartment of the vehicle. The heating system includes a control system that allows a vehicle operator to regulate the quantity and/or temperature of air delivered to the passenger compartment so as to achieve a desired air temperature within the passenger compartment. Cooling fluid from the vehicle's engine cooling system is commonly used as a source of heat for heating the air delivered to the passenger compartment.
The heating system typically includes a heat exchanger fluidly connected to the vehicle's engine cooling system. Warm cooling fluid from the engine cooling system passes through the heat exchanger where it gives up heat to a cool air supply flowing through the heating system. The heat energy transferred from the warm cooling fluid to the cool air supply causes the temperature of the air to rise. The heated air is discharged into the passenger compartment to warm the interior of the vehicle to a desired air temperature.
The vehicle's engine cooling system provides a convenient source of heat for heating the vehicle's passenger compartment. One disadvantage of using the engine cooling fluid as a heat source, however, is that there may be a significant delay between when the vehicle's engine is first started and when the heating system begins supplying air at a preferred temperature. This may occur, for example, when the vehicle is operated in very cold ambient conditions or has sat idle for a period of time. The delay is due to the cooling fluid being at substantially the same temperature as the air flowing through the heating system and into the passenger compartment when the engine is first started. As the engine continues to operate, a portion of the heat generated as a byproduct of combusting a mixture of fuel and air in the engine cylinders is transferred to the cooling fluid, causing the temperature of the cooling fluid to rise. Since, the temperature of the air discharged from the heating system is a function of the temperature of the cooling fluid passing through the heat exchanger, the heating system will generally produce proportionally less heat while the engine cooling fluid is warming up than when the cooling fluid is at a desired operating temperature. Thus, there may be an extended period of time between when the vehicle's engine is first started and when the heating system begins producing air at an acceptable temperature level. The time it takes for this to occur will vary depending on various factors, including the initial temperature of the cooling fluid and the initial temperature of the air being heated. It is preferable that the temperature of the cooling fluid reach its desired operating temperature as quickly as possible.
Another potential limitation of using the engine cooling fluid as a heat source for the vehicle's heating system is that under certain operating conditions the engine may not be rejecting sufficient heat to the cooling fluid to enable the air stream from the vehicle's heating system to achieve a desired temperature. This may occur, for example, when operating a vehicle with a very efficient engine under a low load condition or in conditions where the outside ambient temperature is unusually cold. Both of these conditions reduce the amount of heat that needs to be transferred from the engine to the cooling fluid to maintain a desired engine operating temperature. This results in less heat energy available for heating the air flowing through the vehicle's heating system.